Note: Descriptions are shown in the official language in which they were submitted.
CA 02764656 2011-12-07
Device for Detecting the Length of a Drilling Column
The invention relates to a device for detecting the length of a drilling
column that has a
plurality of pipes assembled on couplings.
One important measured quantity within the framework of deep wells in the
petroleum
and natural gas industry is the depth of the drill hole. It is found from the
current position of
the drill block and the sum of the lengths of the installed pipes and
equipment.
Currently, the depth is determined semiautomatically. The block position is
detected
automatically. Conversely, each pipe installed in the drilling column is noted
manually by
operators. This often results in faults due to the pipes being entered
incorrectly or not at all.
The problem of automatic detection of depth consists mainly in automatic
recording of
the installed pipes. As a result of mechanical/thermal stress on the pipes,
the use of RFIDs, for
example, is a problem. Optical recording methods (for example, a bar code) are
precluded due
to the fouling of the pipe.
Therefore, it is desirable to make available a device with which the length of
the
drilling column can be easily and reliably detected.
This may be achieved by a device of the initially mentioned type in that on
the pipes,
there is at least one electrical line that is galvanically connected to the
couplings, on one end of
the electrical line there being a means for the feed of electrical pulses into
the electrical line
and for detecting the propagation time of the electrical pulses from the means
to a reflection
site that is located, for example, on the other end of the electrical line and
back.
In the device according to the invention, so-called time domain reflectometry
is used
that has been employed for a long time for detecting the length of cables. In
this case, short
rectangular electrical pulses are applied to the line, and reflections on the
cable are detected.
Depending on the terminating resistance (no load, short circuit, matching or
mismatch), echoes
occur from whose time behavior (propagation time) the distance to the cable
end or to a defect
site can be deduced. The prerequisite for the exact determination of the
location is constant
propagation times in the cable. They dictate constant cable properties
(dielectric). The method
is often used to look for line breaks or cable pinches. Also, it is already in
use in the petroleum
industry for these purposes.
The invention is based on pipes that are equipped, for example, with an at
least two-
pole electrical cable or alternatively with a coaxial cable.
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One preferred embodiment of the invention is characterized in that the line of
a pipe is
galvanically connected to the line of a following pipe at the transition point
to the coupling,
and in that the surge impedance of the transition points is matched to the
surge impedance of
the line, and is preferably essentially the same. Because the surge impedance
of the line in the
individual pipes and the surge impedance of the transition points are known or
constant, there
is a constant known relationship between, on the one hand, the length of the
drilling column
and the cable length and, on the other hand, the propagation time of
electrical pulses through
the cable, from which the length of the cable and subsequently of the drilling
column can be
computed.
All pipes are preferably equipped with the same cable (same surge impedance).
The
geometrical structure of the pipe connections and couplings between the pipes
and the
transition points of the electrical line to the couplings is likewise always
the same (same surge
impedance); this greatly simplifies the computation of the length from the
propagation time.
Since the distance from the last cabled pipe to the drilling head is constant
and likewise
known, the length of the drilling column from the feed site of the electrical
signals to the tip of
the drilling head can therefore be easily computed. Thus, using the invention,
the length of the
drill string or the depth of the drilling head can be automatically detected
by way of the cable
length that is changed by the pipes that are added or removed.
In order to be able to definitively deduce the total cable length, on the end
of the line
there must be a reflection site for the electrical signals, for example in the
form of a mismatch.
The latter is present, for example, in the form of a line that is open on the
end in the case of the
cabled and galvanically connected pipes. The incoming wave is strongly
reflected by this open
cable end and can be definitively detected at the cable start or feed site of
the electrical signal.
By means of the measured propagation time, the position of the reflection site
can be deduced
based on the known propagation velocity in the cable.
Since the propagation velocity depends mainly on the cable dielectric, it must
be
constant (same cable, same qunlity). Since the propagation velocity is a
function mainly of the
cable dielectric, it must be constant (same cable, same quality). Since the
cable dielectric is
dependent on temperature, the temperature constitutes an important variable.
By introducing a
depth-dependent correction factor, the effect of temperature on the dielectric
can be
compensated with increasing depth. This correction factor can be assumed to be
a constant
value. It can also be determined, however, by temperature measurements in the
drill string.
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CA 02764656 2011-12-07
This is can [sic] be advantageous since the temperature of the cable depends
not only on the
depth (roughly 3 C/100 m) and the geological properties of the formation, but
mainly on the
temperature of the drilling fluid. The latter, however, changes due to the
above-described
variables or as dictated by different flow rates. Most advantageously, the
temperature is
measured by measurement stations along the cable, and the temperature behavior
is considered
in the depth correction. In the case of the cabled drill string, a
distribution of the measurement
systems along the drill string is preferred. They can also measure the cable
temperature in
addition to data that are relevant to drilling. Knowledge of the temperature
path of the cable
dielectric is also important for these correction methods.
Since the pipes of the drilling column are galvanically connected at regular
intervals
(for example, 9 m or 15 m), a few hundred transition points for wave
propagation are formed
in the drilling column. In order to avoid an adverse effect of these
transition points in the surge
impedance (for example, reflections), the galvanic connection between the
pipes is preferably
made geometrically and electrically such that no impact occurs in the surge
impedance
(matching to the surge impedance of the line) and thus the recognition of the
primary
reflection on the end of the line is not hindered or changed.
The electrical consumers that are present in all cases in the drilling column
string
likewise constitute a transition point in the surge impedance for this method
and can likewise
be matched to the surge impedance of the line by electrical measures. One
additional
possibility consists in placing the consumer at the tip of the drilling
column. In this case, a
mismatch to the surge impedance of the line that is as large as possible must
be produced.
Matchings of the surge impedance (AC voltage resistance) are easily possible
when the
supply line is operated with DC voltage (for example, 400 V DC). For example,
a suitable
capacitor that is connected in parallel to the consumer does not disrupt
operation of the DC
consumer, but for high frequency constitutes a short circuit and implements an
extreme
mismatch.
The pulses are fed into the turning drill string during the drilling process
by means of
slip rings in a so-called swivel, a pivoted part on the top end of the drill
string for the feed of
rinsing fluid, but also electrical energy and communication. Since, during the
round trip of the
column, the swivel is not connected due to the process, the feed can take
place directly
galvanically by a plug that can be mounted on the so-called elevator link, one
part of a hoisting
device that is used for the round trip of the drill string.
3
In order to avoid disrupting HF communication that may be underway with
devices in
the drilling column or drilling head, the depth measurement should preferably
be synchronized
with the data transmission (for example, master/slave) since isolation of the
reflected electrical
signals could otherwise be difficult or impossible.
The invention can eliminate the manual input of pipes and their length since
the depth
can be deduced by means of propagation time measurement. Commercial
reflectometers can be
used and can function as a measurement device. The measured value need be
corrected only by
one offset to the drilling head or by a correction factor (pipe length/cable
length). Since the cable
expands simultaneously with the pipe, this change in length is recognized in
the propagation time
measurement.
In the invention, at the same time, one side effect is also quality control of
the cabled
drilling column. Defective connection sites or line interruptions are
optically detected on the
oscilloscope in servicing or in operation by suddenly incorrect depth
measurement values. The
location of the defect can be deduced by the propagation time.
One important aspect in the use of cabled, galvanically connected drilling
columns is the
reliability of the connection between the pipes. Using the invention, the
quality of the connection
can be immediately checked after the conductive connection is established (for
example,
interruption, short circuit).
One preferred embodiment of the invention is described in more detail below
with
reference to the attached drawings in which a device according to the
invention on a drilling
column is schematically shown.
Two pipes are symbolically numbered 1 in the drawings; they are connected to
one
another using a coupling 2. A drilling column consists of a plurality of these
pipes 1 that are
connected to one another using couplings 2 and of a drilling head that is not
shown in the
drawings and that is attached to the end of the last pipe 1, which end is the
right one in the
drawings. The line phases of a two-pole electrical line are labeled 3 and 4.
The line phases 3, 4
are connected to a power supply 6 on one end via a slip ring 5, and the power
supply transforms
the grid AC voltage into, for example, 400 V DC voltage.
A means 7 for the feed of electrical pulses into the line phases 3 and 4 of
the electrical
line is connected between the power supply 6 and the slip ring 5 and detects
the propagation
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CA 02764656 2011-12-07
time of the electrical pulses from the means 7 or the feed site into the line
phases 3 and 4 to a
reflection site on the other end 8 of the electrical line 3, 4 and back.
On the other end 8 of the line that is shown at the right in the drawings, an
electrical
consumer 9 can be connected that is supplied with current from the power
supply 6. When the
electrical consumer 9 at the same time is to form the reflection site for the
electrical pulses that
are fed from the means 7, it must be made such that it constitutes a so-called
defect site at
which the signals are reflected. Alternatively, it is also possible for the
electrical consumers 9
to be located somewhere on the path between the feed site and the end 8 of the
line 3, 4, and in
this case, care must be taken that the connection of the electrical consumer 9
does not
constitute a defect in order not to adversely affect the propagation time
measurement of the
elechical signals. If there is not a consumer 9 that constitutes a defect on
the end 8 of the line
3, 4, the open end of the line 3, 4 forms the defect on which the signals are
reflected.
Instead of the two-pole electrical cable 3,4 that is shown in the drawings, in
addition
or alternatively, for example, a coaxial cable or another measurement line can
be used that is
used without connection of a power supply 6 and an electrical consumer 9, for
example, only
for measuring the length of the drilling column.
Since the propagation time of the electrical signals in the line 3, 4
increases with each
additional pipe 1 and each additional coupling 2, using the change of the
propagation time, the
length of the drilling column, specifically of the line 3,4, can be
definitively computed,
optionally with consideration of additional lengths such as the length between
the feed site of
the electrical signals and the slip ring as well as the distance from the end
8 of the line 3, 4 to
the drilling head that is not shown in the drawings.
In order to enable an optimum measurement, the following measures are
recommended:
- Matching of the pipe connections to the surge impedance of the line
(connection free
of reflection);
Matching of consumers along the line to the surge impedance (connection free
of
reflection);
Generating a mismatch on the end of the measured distance generally on the end
of the
line (primary reflection);
Mechanical connection of cable and drilling column and cabling of the pipes
with
constant cable length;
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CA 02764656 2011-12-07
Compensation of the material influences on the propagation time measurement
(for
example, temperature dependency of the electrical properties of the cable
(dielectric));
Synchronization of the propagation time measurement with the HF communication
on
the line.
By using different cable types, cable cross-sections or diverse electrical
connectors
(slip rings, cable drums, etc.), geometrical matching to the surge impedance
cannot be possible
from case to case for mechanical reasons (lack of space in the mechanical
connector). In this
case, matching the connectors to the line by an electrical network (T circuit,
H circuit, pi
circuit) with discrete electrical components (matching network) can be
achieved. Since these
matching networks likewise consume electrical energy, an independent
measurement line can
alternatively be installed that does not use the energy supply.
The electrical consumers that are present along the drill string for the
measurement
method that is preferably to be used in the invention likewise constitute a
transition point in
the surge impedance and are matched to the surge impedance of the line by
electrical
measures. For this purpose, transformation networks can be built up from
discrete electrical
consumers that match the impedance of the consumer to the surge impedance of
the line.
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